This invention relates to the fabrication and structure of electrocatalyst coated 3-dimensional porous high surface area electrode structures for use in electrolytic cells for a variety of electrochemical production processes as anodes or cathodes. More particularly, this invention relates to the fabrication and structure of electrocatalyst coated high surface area porous type electrode structures fabricated from fine metallic and/or conductive ceramic oxide composition fibrous materials.
High surface area electrodes are finding increasing use in recent years in various electrochemical processes. This is because of new advances in material processing science in the preparation and manufacture of high surface area metallic and electrically conductive inorganic substrates as well as due to the increasing need for high selectivity electrodes to achieve higher conversion efficiencies in electrochemical processes.
There are several types of commercially available high surface electrodes on the market today. These are generally made from graphite in the form of felts, foams and woven structures. In general, the felts are made from fine, short fibers that are mechanically interlocked. A problem with graphite is that it is not as conductive as metals and that there are problems with producing an adequate electrical or physical bond between the graphite material and a current distributor. In addition, significant areas of the felt structure may not participate in the electrode reactions because of minimal mechanical/electrical contact between the fibers because of their short lengths. These fibers have length to diameter ratios that are generally less than 1000:1. These graphite structures are also generally limited to operation at low cell current densities because of the low conductivity of graphite in combination with the minimal graphite inter-fiber contacts within the structure. In addition, graphite is not generally stable as an oxygen generating electrode.
Metallic materials are also now available prepared from copper, nickel and stainless steels and their alloys. One material type is in the form of a metallic foam product with specifications in terms of pores per inch (PPI). These materials range from 10 to 300 PPI, but the actual active specific surface area is generally below 30 cm.sup.2 /cm.sup.3. In addition, the metallic foams have mechanical properties that can range from being very hard and incompressible to very fragile and brittle. In addition, electrode structures may be prepared from sintering fine powders of these metals, but the density of these materials is generally limited to about 60% or greater, which greatly increases the hydraulic pressure drop through the structure, making it uneconomical or impossible to operate without employing very high pressure rated electrochemical cell designs.
Metallic felts prepared from fibers are also now becoming available, but these are generally prepared from stainless steels using small short fibers with length to diameter aspect ratios that are considerably less than about 1000:1. These felts are made by air-laying or wet filtration methods, and cannot be made by these methods using fibers with larger diameter to length aspect ratios. Woven stainless steel materials are also available made from the fine diameter wires or tow fiber bundles containing multiple filaments. Since these woven type structures use continuous length filaments, the length to diameter aspect ratio is greater than 1000:1. These stainless steel woven materials are themselves very conductive, as are their surfaces, and there is no problem with fiber to fiber conductive paths in the structure because of this conductivity.
In the case of valve metal woven wire constructions, for example titanium, the conductive paths through just the long wire lengths are not adequate for an even distribution of the current throughout the structure. The woven material to be used as an effective 3-dimensional high surface area electrode structure also requires a fiber to fiber electrical contact, which depends on the fiber surfaces and their corresponding areas being conductive and intimately in contact with each other. Since valve metals form protective nonconductive oxide films on their surfaces, these conductive contact points may not be stable in the electrochemical system and form nonconductive oxides, and the material will then not be suitable as an electrode. Also, woven materials, both made from either stainless steel or valve metals, have been observed to not be suitable as electrode structures in electrochemical cells for operation at current densities greater than about 1 to 2 KA/m2. One explanation is that the 3-dimensional electrical conductivity of the structure relying on a mechanical fiber to fiber contact is not adequate above this range, resulting in a substantially higher cell electrode operating voltage with corresponding changes in the competitive electrochemical reactions occurring at the electrode surfaces. Another explanation for inadequate performance of woven structures made from multi-filament strands (or tow bundles) is that the porosity of these structures is non-uniform, such that the zones with highest surface area do not allow penetration of current through the electrolyte between closely spaced fibers.
The technology for the processing and production of valve metals, such as titanium, in the form of fine wire, filaments and tow fiber is now available. The problem is in fabricating the filamentary valve metal raw material into a form that is suitable as a 3-dimensional, uniformly conductive high surface area electrode structure and developing methods for the application of an even, economical amount of an active electrocatalyst material onto the structure. In addition, a method for efficiently and evenly distributing electrical current to the structure is also required to be suitable for an electrochemical process. The higher the effective surface area of the electrode structure, with a uniform distributed current density, the higher the single pass conversion efficiency performance of the electrode for the specific electrochemical process application.